An increasing number of applications are seeking navigation solutions that do not rely on Global Navigation Satellite Systems (GNSS) for periodic aiding. Personal indoor navigation and urban navigation are two examples.
Typically navigation aids rely on one or both of two types of navigation aids, incremental and absolute. Incremental navigation is the ability to determine position and attitude with sensors that measure incremental motion. Sensors such as gyroscopes, accelerometers, and odometers are used to mechanize incremental navigation systems. Absolute navigation is the ability to determine position and attitude with sensors that measure the location directly. Sensors such as GNSS, magnetometers, and radio beacons are used to mechanize absolute navigation systems.
The utility of incremental navigation is affected by drift in the localization estimate. In many applications, localization would need to be drift-less to be useful. Hence, many applications use absolute navigation sensors to aid the incremental navigation sensors. Most absolute navigation sensors, however, such as GNSS, need infrastructure to operate properly, and therefore are not guaranteed to be available under some key environmental and terrain conditions. In some cases, both GNSS and magnetometer performance deteriorates in similar environments, leading to a ‘black out’ in absolute localization and aiding. Examples of such environments include indoors, urban canyons, and under foliage located over a ferro-magnetic structure.
The above issues with localization aiding have prompted many people to develop alternative localization sensors and methods. Examples of such sensor systems are stereo and monocular vision systems, Light Detection and Ranging (LIDARS), and RADARS. Such sensors have been used primarily as incremental navigation systems. There are also an increasing number of prototype systems that detect signals of opportunity such as Wi-Fi and Radio that use triangulation/trilateration to localize a platform. Such systems have also been used as absolute navigation systems.